There is a considerable demand for antennas that have a dual band performance and a tunable capability for operating in different frequency bands. For example, in wireless communications, the GSM standard used primarily in Europe has frequency bands of 890-915 MHz and 935-960MHz for the uplink and downlink, respectively. In addition to this system, the new generation of personal communication system (PCS), such as DCS 1800, has frequency bands of 1.710-1.785 GHz and 1.805-1.880 GHz for the uplink and downlink, respectively. Hence, for a portable hand-phone to be compatible with the two systems (GSM and PCS) the antenna should be able to operate in these two bands. However, communication standards vary across geographical regions. In North America, the Interim Standard-54 (IS-54) is used instead of the GSM standard. It occupies frequency bands of 869-894 MHz for the uplink and 824-849 MHz for the downlink. The antennas needed for a hand-phone that is useable in both Europe and North America will now be required to cover the three different communication standards.
The prior art suggests that this could probably be achieved with multiple antennas or a manual extractable antenna. In most cases, a single plane antenna is preferred. Most of the prior art tunable antennas use diodes or shorting pins to achieve the tuning performance. This additional circuitry adds protrusion and complexity to the antenna structure that limits the capability to operate in a compact, conformal and rugged environment.
The use of ferroelectric material in phase shifters is described in "Ceramic Phase Shifters for Electronically Steerable Antenna Systems", Varadan et al., Microwave Journal, January 1992, pages 116-126. Ferroelectric materials have also been described for use in electronic phased scanning periodic arrays. For example such arrays are described in U.S. Pat. No. 5,589,845 to Yandrofski et al., U.S. Pat. No. 5,729,239 to Rao and U.S. Pat. No. 5,557,286 to Varadan et. al. In such arrays, scanning is achieved by positioning array elements in a linear broadside arrangement. Energy coupling occurs in the horizontal azimuth plane. The common dielectric constant values for Barium Strontium Titanate materials used in the system of the Varadan et al. patent or in the system disclosed in U.S. Pat. No. 5,427,988 to Sengupta et al. are relatively high for typical antenna applications.
Microstrip antennas with high permittivity substrates have suffered from poor efficiency and narrow bandwidth. The stacking of director elements could enhance the gain and bandwidth and introduce dual band performance. U.S. Pat. No 4,162,499 to Jones, Jr. et al. and U.S. Pat. No 5,561,435 to Nalbandian et al. suggest stacking of antennas. However, the antennas of these patents are optimized at discrete frequencies only, impeding their use for frequency hopping communication systems.
Accordingly, there is a need for a technology and for a single antenna to meet multi-usage and multi-frequency requirements. There is also a need for such antennas to have a planar structure that is flexible enough to conform to hand phone or other wireless device constructions.